1. Field of the Invention
This invention relates to an electrochemical cell and more particularly refers to a new and improved electrochemical storage cell or battery based on alkali metal and sulfur with at least one anode chamber and at least one cathode chamber separated from each other by an ion-conducting solid electrolyte.
2. Description of the Prior Art
An electrochemical storage cell or battery based on alkali metal and sulfur having at least one anode chamber and one cathode chamber separated from each other by an ion-conducting solid electrolyte, and a felt fabric of graphite or carbon contained in the cathode chamber in addition to the sulfur is known in the art as shown by German Published Prosecuted Application No. 1,671,760.
The purpose of the felt fabric is to increase the boundary surface necessary for the electrochemical reaction, between the sulfur or the alkali polysulfide produced during the discharge, and the cathodic current collector, of which the felt fabric can be considered as an extension. In addition, this makes the distance between the electrolyte and the cathodic current collector, extended in this manner, small, so that the resistance of the sulfur or the sodium polysulfide contributes less to the cell's internal resistance.
An important disadvantage of such a cell is, however, that it can be only partially recharged with large currents. In a Na/S cell, for instance, this disadvantage is related to the fact that sulfur (with small percentages of dissolved Na.sub.2 S.sub.5) and Na.sub.2 S.sub.5 (with percentages of dissolved sulfur) form two immiscible liquids at the operating temperature of such a cell, which is between 300.degree. and 350.degree. C. For, if a discharged Na/S cell, which contains Na.sub.2 S.sub.3 in the cathode chamber, is recharged, then sulfides with more sulfur are formed first until Na.sub.2 S.sub.5 has been produced in the entire cathode chamber or at least locally. Upon further charging, liquid sulfur is then produced at the points with a high electrochemical reaction rate. The sulfur being an insulator, blocks the electrochemical processes at these points.
As the electrochemical processes in a graphite felt of homogeneous structure, i.e. substantially the same pore radius, conductivity etc. independent of the distance from the solid electrolyte, take place in the vicinity of the electrolyte wall, sulfur is formed preferentially in the vicinity thereof. When the entire electrolyte surface is covered with sulfur, the entire amount of Na.sub.2 S.sub.5 in the rest of the cathode chamber cannot contribute any more to the electrochemical reaction. This then reduces the capacity of such a cell to such an extent that the advantage of a high energy content, which exists in principle, is largely lost.
In storage cells with high current densities, such as are being planned for the propulsion of electric vehicles and for covering peak loads in electrical networks, however, it is particularly important that recharging as rapidly and completely as possible be feasible.